WO1996021757A1

WO1996021757A1 - Rotatable susceptor with integrated ferromagnetic element

Info

Publication number: WO1996021757A1
Application number: PCT/US1995/016945
Authority: WO
Inventors: Charles Su-Chang Tsai
Original assignee: Tsai Charles Su Chang
Priority date: 1995-01-09
Filing date: 1995-12-26
Publication date: 1996-07-18
Also published as: JP2001523391A; US5468299A; DE69524640T2; ATE210747T1; DE69524640D1; EP0871804B1; CA2206828A1; EP0871804A1

Abstract

A device comprising a flat susceptor (14) rotating parallel to a reference surface (30) about a rotary shaft (20) perpendicular to this surface and comprising means for obtaining the stability of the susceptor held in sustentation and means for obtaining its rotary movement, is characterized in that these means are constituted by the integrated ferromagnetic elements (11) in the susceptor, by the rotary shaft, by the reference surface with grooves or depressions (60, 62), by a magnetic actuating assembly of magnetic solenoids (74), by a gas inlet (40), by control electronics (80), and by optional sensors (82).

Description

ROTATABLE SUSCEPTOR WITH INTEGRATED FERROMAGNETIC ELEMENT

Background

1. Field of The Invention

The invention relates to a device comprising a flat susceptor rotating parallel to a reference surface about a shaft perpendicular to this surface and comprising means for obtaining the stability of the susceptor held in sustentation and means for obtaining and measuring its rotary movement.

The invention further relates to a reactor chamber for vapor-phase epitaxy or chemical-vapor deposition provided with such a device. The invention can be used in the manufacture of flat rotatable sample carriers for reactor chambers for vapor-phase epitaxy or chemical-vapor deposition, more particularly for the epitaxial growth from the vapor phase of layers of compounds of the III-V or II- VI group for forming semiconductor devices.

2. Description of Prior Art

The use of rotating sample carriers in the field of epitaxial growth from the vapor phase of the compounds of the HI-V group is known from the publication entitled "Multi-Wafer Growth of Extremely Uniform GaAs Layers by Organometallic Vapor Phase Epitaxy" from the Electronic Materials Conference, University of Colorado, June 19-21, 1985, by S. Komeno, H. Tanaka, I. Itoh et al. This publication shows that it is important during the growth of epitaxial layers from the vapor phase to rotate the sample in order to obtain uniform layers and to form components of high quality, such as field effect transistors, semiconductor diode lasers and many others.

Vapor-phase epitaxy imposes stringent requirements on the materials used within the reactor chamber. High temperatures of the order of 900°C are encountered routinely. High purity environment that is as particle-free as possible is required for the growth of high-quality and low-defect-density epitaxial layers. Highly-reactive and acutely-toxic gas ambients within the reactor chamber dictate the use of inert materials and leak-proof seals. The nermal variations across a susceptor, the difference in rotation rates of different susceptors, the rotation-rate variations from growth to growth, and the gas-flow disturbances and inhomogeneities must be kept to an absolute minimum to obtain highly-uniform epitaxial layers suitable for mass-production.

Susceptor rotation by external motor with mechanical feedthroughs is met with difficulties. The mechanical members traversing the reactor-chamber wall enclosing the acutely-toxic gases require leak-proof mechanical seals operating at high temperatures Extremely complicated mechanical gearing mechanism for rotating susceptors in the planetary form is difficult to implement, especially when using the non-lubπcated refractory matenals suitable for the high-temperature, highly-reactive, and acutelv-toxic gas ambients

Susceptor rotation by gas flow in U S Patent No 4,860,687 to Fπjhnk, 1989 August 29, was a significant improvement over rotation by external motor with mechanical feedthroughs Two commercially-available devices are illustrated in two prior-art drawings of FIGS la and lb These gas-flow rotation devices eliminated the need for mechanical feedthroughs and high-temperature mechanical seals A viable planetary form of susceptor rotation was also demonstrated

In FIG la, a susceptor 14 rotates parallel to a reference surface 30 about a rotary shaft 20 perpendicular to reference surface 30 Susceptor 14 is held in sustentation and caused to rotate in the indicated direction by the action of gas flow through several gas inlets 40α, 40b, and 40c into several helical grooves 51α, 5lb, and 51c on reference surface 30 The rotary movement is obtained by a force ot viscosity of the gas

FIG lb illustrates the prior-art planetary form of susceptor rotation by gas flow A main susceptor 14 rotates in the indicated direction Main susceptor 14 also includes a number of secondary susceptors 114, 214, and 314 on a reference surface 130 facing away from reference surface 30 Secondary susceptor 114 is used as an example applicable to secondary susceptors 214 and 314 Secondary susceptor 114 is held in sustentation and caused to rotate in the indicated direction by the action of gas flow through an additional gas inlet 42 in reference surface 30, a system of gas conduits 43, and several gas inlets 140α 140b, and 140c into several helical grooves 151-i, 151/7, and 151c on reference surface 130 Similarly, the rotary movement is obtained by a force of viscosity of the gas

Despite its significant improvement over rotation by external motor with mechanical feedthroughs, susceptor rotation by gas flow still suffers from many disadvantages as listed below

(a) The allowable range of rotation rates is limited by the design of grooves 51α, 51fc,and 51c, and the available gas-flow range The minimum rotation rate is constrained by the minimum gas flow required for susceptor sustentation The maximum rotation rate is constrained by the increase in the gap between the undersurface of susceptor 14 and reference surface 30 with increasing gas flow, where more and more gas flow bypasses grooves 51α, 5lb, and 51c enUrely, resulting in little or no increase in the rotation rate

(b) High rotation rates require high gas-flow rates The large amounts of escaping gases from the gas-flow rotation schemes disrupt significantly the flow of gases in a reactor chamber, causing inhomogeneity and turbulence in the gas flow over the susceptor, resulting in inhomogeneous and defective epitaxial-growth layers

(c) Rotation rates of susceptors 14, 114, 214, and 314 are difficult to measure in a coated reactor chamber, and are sensitive to the individual machining tolerances of gas inlets 40α, 40b, 40c, 140α, 140/ , and 140c, system of gas conduits 43, and grooves 51α, 51i>, 51c, 151α, 151 , and 151c Perfect matching of rotation rates between individual secondary susceptors 114, 214, and 314 in the planetary device is extremely difficult, because the individual rotation rates are not adjustable

(d) The failure or rate change in the susceptor rotation due to clogging in gas inlets 40α, 40/ , 40c, 42, 140α, 140fc, or 140c, or in system of gas conduits 43, or due to friction-producing dust particles are not directly detectable except by visual inspection, which is quite difficult to perform with heavily-coated reactor-chamber walls (e) The process of starting and stopping rotation generates contaminating particles due to the frictional contact between rotating susceptor 14 and reference surface 30 when the gas-flow inputs start or stop.

(f) Gap between the undersurface of susceptor 14 and reference surface 30 varies with rotation-rate changes. Increasing the rotation rate requires gas-flow increase, which increases the gap. The changes in the gap cause growth temperature variations and produce undesirable effect on epitaxial growth.

(g) At constant mass-flow rates of gas input, the rotation rates of susceptors 14, 114, 214, and 314 vary with the growth temperature and pressure due to changes in the gas viscosity and volume. The calibration of gas-flow rates using each combination of growth temperature, reactor chamber pressure, and rotation rate is required.

(h) Synchronization of susceptor rotation with source-gas switching during epitaxial growth is extremely difficult. Not synchronizing rotation with gas switching results in inhomogeneities in epitaxial-growth layers across the susceptor. This is especially critical for thin layers with layer thicknesses comparable to atomic dimensions.

(i) Angular orientation and position of each susceptor 14, 114, 214, or 314 are not directly detectable except by visual inspection. In an automated production machine, the unsupervised loading and unloading of indexed wafers in a reactor chamber require machine-accessible data on the orientations and positions of individual susceptors.

Objects and Advantages

It is accordingly an object of the present invention to eliminate the above disadvantages of susceptor rotation either by external motor with mechanical feedthroughs or by gas flow. Several objects and advantages of the present invention are:

(a) No constraint on minimum rotation rate. The maximum rotation rate is mainly constrained by the design of the magnetic structures and the control electronics.

(b) Escaping gases that disturb the gas flow over the susceptor are minimized. Gas flow is required only for susceptor sustentation.

(c) Rotation rates of susceptors can be determined essentially exactly. Perfect matching of rotation rates between individual susceptors in the planetary device is an inherent characteristic. Different adjustable rotation rates for different susceptors in the planetary device are also possible.

(d) Failures or rate changes in the susceptor rotation due to external influences can be quickly detected with the optional sensors, thereby allowing for possible corrective actions to be taken by the control electronics or a human operator.

(e) The process of starting and stopping rotation generates no contaminating particles, because sustentation of susceptor is independent of the rotation mechanism.

(f) Synchronization of rotation with external events, such as source-gas switching during an epitaxial growth, is simple.

(g) Relative angular orientation and position of each susceptor are known at almost all times with the optional sensors, thereby allowing for device incorporation into automated processing machines.

Other objects of the invention are to provide versatile rotatable platforms useful outside of the field of vapor- phase epitaxy. The rotatable platform, with or without the substitution of the sustentation means with other bearing means, would allow for extremely complicated rotation patterns with multiple rotation axis without any complicated mechanical gearing or motor systems. The rotatable platform could also be used in a liquid environment, since the principles for its operation do not depend on the specific properties of the gas phase.

Further objects and advantages of the invention will become apparent from a consideration of the drawings and ensuing description.

Description of Drawings

FIG. la is a prior-art exploded and perspective view of a device comprising a susceptor provided with a rotary shaft and a system for setting into rotation in which a force of viscosity of a gas is applied,

FIG. lb is a prior-art exploded and perspective view of a device permitting the planetary rotation of several susceptors,

FIG. 2a is a sectional view of a basic device according to the invention,

FIG. lb is a plan view of the susceptor according to FIG. 2a,

FIG. 2c is a plan view of the reference surface and the magnetic actuating assembly according to FIG. 2a.

FIG. 3a is a perspective view of a second embodiment of a susceptor according to the invention,

FIG. 3 is a perspective view of a third embodiment of a susceptor according to the invention,

FIG. 3c is a plan view of a fourth embodiment of a susceptor according to the invention,

FIG. 4a is a sectional view of a second embodiment of a reference surface for susceptor sustentation according to the invention,

FIG. 4b is a plan view of the reference surface according to FIG. 4a,

FIG. 4c is a sectional view of a third embodiment of a reference surface for susceptor sustentation according to the invention,

FIG. 4d is a plan view of the reference surface according to FIG. 4c,

FIG. 4e is a sectional view of a fourth embodiment of a reference surface for susceptor sustentation with an alternative rotary shaft according to the invention,

FIG. 4/is a plan view of the reference surface according to FIG. 4e,

FIG. 5 is a perspective view of a rotating susceptor actuated by two external permanent-magnet rotors mechanically connected to a motor according to the invention,

FIG. 6a is a plan view of an embodiment of a device according to the invention,

FIG. 6b is a plan view of an alternate embodiment of a device according to the invention,

FIG. 6c is a sectional view of the device according to FIG. 6b,

FIG. la is a perspective view of a magnetic flux sensing device based on the Hall effect,

FIG. 7b is a perspective view of a magnetic flux sensing device based on the generation of electromotive force in a coil by magnetic induction,

FIG. 7c is a perspective view of a metal sensing device based on the changes of coil inductance in the presence of metal,

FIG. 8α is a graph of electrical currents I_A, I_B, and I_c applied to different magnetic solenoids versus time for one revolution of susceptor rotation,

FIG. Sb is a graph of electrical currents I_A, I_B, and I_c applied to different magnetic solenoids versus time for one revolution of susceptor rotation, with improved actuation characteristics,

FIG. 9 is an exploded and perspective view of a preferred embodiment of a device according to the invention,

FIG. 10a is an exploded and perspective view of a device permitting the planetary rotation of several susceptors according to the invention, not showing the magnetic actuating assembly,

FIG. I0b is a sectional view of the device according to FIG. 10a,

FIG. 10c is a plan view of the device according to FIG. 10a.

Description and Operation of The Invention

1. Basic Device

In the following description, a platform in the form of a disk will preferably be chosen, although other platforms of different shapes or structures can be utilized equivalently. In addition, a platform is equivalent to a stage, a pedestal, a support, a prop, or a stand.

As shown in FIGS. 2a, 2b, and 2c. the basic device according to the invention comprises one or more ferromagnetic elements llα, lib, lie, and lid of unmagnetized or magnetized ferromagnetic material, such as cobalt (Co), iron (Fe), nickel (Ni), or the alloys of Co, Fe, Ni, and/or other materials, integrated into a platform or a susceptor 14 rotating parallel to a reference surface 30 about a shaft 20 perpendicular to this surface.

According to the invention, platform 14 is held in sustentation above the reference surface 30 with a gap h>0 at all points between the undersurface of platform 14 and reference surface 30 by the action of gas flow through one or more gas inlets 40. Platform 14 is induced to start rotating, to continue rotating, and/or to stop rotating about shaft 20 by the magnetic interactions between ferromagnetic elements lla, lib, li , lid and an assembly of magnetic actuating means 70. In addition, actual relative angular orientation and rotation rate of platform 14 can be measured if sensor means are included as a part of assembly 70.

Although not part of the device according to the invention, portions of a reactor-chamber wall 90 are also shown to illustrate the incorporation of the device into a reactor chamber, where assembly 70 is shown positioned outside of the reactor chamber.

In FIGS. 2b and 2c, the edge 16 of platform 14 is also drawn to aid understanding and visualization.

Platform 14 may be formed from any kind of hard materials of low deformation, inclusive of polymers.

For use in an epitaxy chamber, these materials must moreover be refractory. These materials may be: graphite, metals, ceramic materials, crystalline materials, such as silicon, gallium arsenide, sapphire.

All of these materials may be used for platform 14, for shaft 20, as well as for reference surface 30.

Although specific devices comprising susceptors are illustrated, devices comprising platforms are implied in the ensuing description.

2. Ferromagnetic Elements in Susceptor

In the following description, the number of ferromagnetic elements in a susceptor is chosen to be four, for example, although any number one or greater is possible depending on specific application.

There are many possible shapes and structures for the ferromagnetic elements integrated in susceptor 14. As shown in FIGS. 2a and 2b, four ferromagnetic elements llα, lib, li , and lid, cylindrical in shape with diameters smaller than the thickness of susceptor 14 and lengths smaller than the radius of susceptor 14, are incorporated at edge 16 of susceptor 14, where the positions are deduced from each other by rotations of 2π/n, where n=4 in this case.

As shown in FIG. 3a, four ferromagnetic elements llα, lib, lie and lid, composing an assembly 11, are also cylindπcal with diameters smaller than the thickness of susceptor 14, but with lengths essentially equal to th radius of susceptor 14, allowing for more efficient pathways for magnetic flux through susceptor 14 Positions of the elements are deduced from each other by rotations of 2π n, where n=4 in this case

As shown in FIG 3b, a continuous structure 11 contains four spokes llα, lib, li , and lid. which are the ferromagnetic elements Positions of the spokes are deduced from each other by rotations of 2π/n, where n=4 in this case This structure resembles the cross section of a rotor in a direct-current brushless electrical motor Structure 11 allows more efficient pathways for magnetic flux through susceptor 14 and distinct elements for magnetic actuation

The ferromagnetic elements may be integrated into susceptor 14 by numerous methods They can be pressed into tight-fitting holes drilled in susceptor 14 They can be shaped with external threads by machining to fit into correspondingly threaded holes in susceptor 14 They can be included in a mold or a cast as solid elements if susceptor 14 can be injection-molded or cast at a lower temperature than the melting point of the ferromagnetic material utilized They can be placed in a sandwich between the top half and the bottom half of susceptor 14 before the two halves are fused together by sintering or by adhesive means Many other integration methods are possible

FIG 3c illustrates the flexibility in the allowed shape of ferromagnetic elements, where four lumps llα, lib, lie, and llα^* are distributed radially along the edge of susceptor 14 and spaced approximately from each other by an angle of 2π/n, where n=4 in this case In this example, the ferromagnetic elements may be formed from ferromagnetic powder molded or cast along with the susceptor in a mold with defined magnetic regions, either as a part of the mold or exterior to the mold, where the ferromagnetic powder preferentially converges and forms individual ferromagnetic lumps

Susceptors shown in FIGS 23 and 3c are preferred for vapor phase epitaxy, because the ferromagnetic elements are placed at the edge of the susceptor in a section not intended to receive a sample, thereby minimizin the temperature variations in the susceptor induced by the different thermal conductivity of the ferromagnetic material

The choices available for the ferromagnetic material for the ferromagnetic elements are restricted by the hig temperatures of the order of 900°C encountered in a reactor chamber The Curie points, the temperatures above which individual materials lose ferromagnetic properties, dictate the choice of the ferromagnetic materials Cobalt (Co) with Cune point of - 1131°C is most preferred along with alloys of Co and other materials Iron (Fe with Curie point of -770°C is useful for susceptors encountering lower temperatures

The ferromagnetic elements may be magnetized or unmagnetized In a reactor chamber, unmagnetized ferromagnetic elements are preferred, because magnetized material quickly changes or loses its permanent magnetic properties when subjected to high temperatures or repeated thermal cycles between room temperature and high temperature 3. Susceptor Sustentation

As shown in FIG. 2α, the weight of susceptor 14 is sustained by the gas-pressure difference between the undersurface and the top surface of susceptor 14 with a gap h>0. This pressure difference is the result of resi nee encountered by the gas flowing from gas inlet 40 to edge 16 of susceptor 14. An equilibrium state is reached when gap h results in the correct amount of resistance encountered by the flowing gas to produce the total gas-pressure difference between the undersurface and the top surface of susceptor 14, when integrated with respect to area over the entire susceptor, that is equal to the weight of susceptor 14 and any weight supported by susceptor 14, for example an epitaxy sample. Gas inlet 40 is placed as close as possible to the center of susceptor 14 at shaft 20 to allow for uniform distribution of gas flow radially.

As shown in U.S. Patent No. 4,860,687 to Frijlink, the basic configuration shown in FIG. 2α for susceptor sustentation does not allow large eccentricity of the weight supported by susceptor 14. Support for susceptor 14 is concentrated in the center of susceptor 14, where the domain of high gas-pressure difference between the undersurface and the top surface of susceptor 14 is centered. This configuration can be used with symmetric loads such as a circular disk or wafer or a concentrated load at the center of susceptor 14. More detailed discussion on this specific subject can be obtained from the above referenced patent.

Several improved systems of susceptor sustentation according to the invention are shown in FIGS. 4α, 4b, 4c, 4α\ 4e, and 4/. The grooves are preferentially applied to reference surface 30 instead of the undersurface of susceptor 14 to minimize temperature variations in susceptor 14. It is known to those skilled in the art that if structures such as grooves are provided in the back surface of the sample carrier, the thermal image of these structures may appear on the finished epitaxial layers in the form of homogeneity defects.

In FIGS. 4α and 4b, a gas inlet 40 supplies gas flow to an annular groove 62 that distributes the flow radially outward into three or more sector-shaped depressions or grooves 60α, 60b, and 60c. Positions of the grooves are deduced from each other by rotations of 2π k, where k=3 in this case.

In FIGS. 4c and 4a^*, three or more gas inlets 40α, 40b, and 40c supply individual gas flows to three sector- shaped depressions or grooves 60α, 60b, and 60c. Positions of the grooves and gas inlets are deduced from each other by rotations of 2π/k, where k=3 in this case.

FIGS. 4e and 4/ show a possible realization of an alternative form of a shaft 20 by a cut profile of a reference surface 30 and a complementary profile on the undersurface of susceptor 14. In this case, the cut profile is that of a circular cone truncated at the tip. A gas inlet 40 supplies gas flow to three sector-shaped depressions or grooves 60α, 60b, and 60c additionally formed on reference surface 30. Positions of the grooves are deduced from each other by rotations of 2π/k, where k=3 in this case.

This form of sustentation does not require a separate shaft 20. However, manufacturing such a device requires precise machining and difficult shaping procedures. Therefore it is considered as preferable to choose a localization of susceptor 14 by cylindrical rotary shaft 20.

These three structures achieve improved stability in susceptor sustentation by widening and equalizing the domains of high pressure-differential between the undersurface and the top surface of susceptor 14. These domains of high pressure-differential correspond approximately to where grooves 60α, 60b, 60c, and 62 are located. The transverse dimension of the grooves may be from one-tenth of a millimeter to several meters and their depth may be from ten micrometers to several centimeters.

Many other equivalent structures for susceptor sustentation, including one or more gas inlets and one or more grooves or depressions of various structures, shapes, or profiles can be made by those skilled in the art. 4 Magnetic Actuating Assembly and Operation of Complete Device

Magnetic actuating assembly 70, shown in FIGS 2α and 2c as boxes, will be described in this section

FIG 5 shows a non-preferred magnetic actuating assembly according to the invention Two permanent magneUc rotors 72 with four poles 72α, 12b, 12c, and 72d are caused to rotate in the indicated directions by a system 84 of mechanical transmission means driven by motor means The polarities of poles 72α, 12b, 12c, and 72d are indicated by the letters N and S Possible locations for sensor means 82α, 82έ>, and 82c are also indicated A susceptor 14 according to FIG 3α is drawn to illustrate the operauon Rotors 72 cause susceptor 14 to rotate at the same rate in the indicated direction The polarities of ferromagnetic elements llα, lib, lie, and lid are also indicated, if they are to be magnetized

Two magnetic rotors are used at equal distance from susceptor 14 to balance the magnetic forces exerted on susceptor 14 If only one rotor is used, there would be strong lateral forces applied to susceptor 14 in addition to the forces necessary to induce susceptor rotation Rotors 72 are placed in the plane of susceptor 14 on opposite ends of a diagonal through susceptor 14 to prevent the application of undesirable vertical forces to lift susceptor 14 or to put additional loads on susceptor 14

Additional control electronics 80 and sensor means 82α, %2b and 82c can be added to monitor the actual rotation of susceptor 14 and to provide feedback for the confirmation of rotation rate and angular position of susceptor 14

The preferred magnetic actuating assembly is shown in FIGS 6α 6b, 6c, and 9 consisting of two or more magnetic solenoids 74α, 14b, 14c, 14d, 14e, and 74/ arranged radially from the center of susceptor 14 Possible locations for sensor means 82α, 826, 82c, 82d, 82e, and 82/ are also indicated Also shown are reference surface 30 and reactor-chamber walls 90 Ferromagnetic elements llα, l b lie, and lid and susceptor sustentation means have been included for completeness and have been described in previous sections

As shown in FIGS 6α and 9. magnetic solenoids 74α, 14b, and 74c are arranged on one side of susceptor 14 and solenoids 74d, 74e, and 74/ on the other side The solenoids are approximately cylindπcal in shape The angular spacing between solenoids on each side is determined by the formula Θ=2π/(n j), here n is the number of ferromagnetic elements on susceptor 14, and j is the number of magnetic solenoids on each side For the case under discussion, Θ is π/6 The angular spacing Θ results in the efficient placement of magnetic solenoids on the outside of reactor-chamber walls 90, which is not shown in FIG 9

As shown in FIGS 6b and 6c, magnetic solenoids 74α, 14b, 14c, 14d, 74c, and 74/ are arranged radially around a circular-reactor-chamber wall 90 Positions of the solenoids are deduced from each other by rotations o 2π/j, where j=6 in this case The solenoids are shaped like the letter L to create a flush surface in the same plane as the top surface of susceptor 14 This illustrates the adaptability of the invention to many different reactor chamber designs Other arrangements of magnetic solenoids can be generated based on the information presente here and the specific application of the device

Pairs of magneuc solenoids are used at equal distance from susceptor 14 to balance the magnetic force exerted on susceptor 14 If singular magnetic solenoids are used instead of pairs, there will be strong lateral forces applied to susceptor 14 in addition to forces necessary to induce susceptor rotation Magnetic solenoids are placed in the plane of susceptor 14 on opposite ends of a diagonal through susceptor 14 to prevent the application of undesirable vertical forces to lift susceptor 14 or to put additional loads on susceptor 14 To cause rotation of susceptor 14 in the direction indicated by R, individual magnetic solenoids marked with letters A, B, or C on FIGS. 6α, 6b, or 9 are supplied with the appropriate current waveforms I_A, I_B, or I_c as shown in FIG. 8α, where the currents change between 0 and m abruptly. The value of m for a particular device can be readily determined by those skilled in the art. The resulting rotation will have observable rotation steps with sudden acceleration and deceleration of rotation rates. The sequence for one revolution of susceptor 14 is indicated.

With the improved waveforms I_A, I_B, and I_c as shown in FIG. $b, where the currents change between 0 and m smoothly, the resulting rotation will be much smoother with little or no jumps in rotation rates. These current waveforms may be produced electronically either by linear drive, where smooth linearly-varying voltages are applied to the solenoids, or by pulse-width-modulated drive, where trains of pulses of constant height in voltage of varying width and/or duty cycle are applied. To reverse the direction of rotation, just apply waveform I_A to solenoid C and waveform I_c to solenoid A.

Control electronics 80 and sensor means 82α, 82b, 82c, 82d, 82c, and 82/ can be added to monitor the actual rotation of susceptor 14 and to provide feedback for the confirmation of rotation rate and angular position of susceptor 14.

Several possible sensor means are shown in FIGS. 7α, lb, and 7e. FIG. 7α shows a magnetic-flux sensing device based on the Hall effect produced on a semiconductor chip, where the magnetic flux as indicated by Φ is converted relatively proportionally into an output voltage. FIG. lb shows a magnetic-flux sensing device, consisting of coils of electrical conductors, that generates an output voltage proportional to the time-rate change in the magnetic flux indicated by Φ. FIG. 7e shows the same coil used in a different way to sense metal by measuring the coil-inductance changes induced by the presence of metal.

These sensors can be placed at the specified locations to detect the presence of ferromagnetic elements llα, l b, lie, or lid to confirm the relative angular position or rotation rate of susceptor 14. Relative angular position can be determined because the position of ferromagnetic elements are known when the susceptor is made. The rotation rate can be determined simply by counting the number of elements detected in a given amount of time or by measuring the time elapsed between detected elements. These two measurements can be made as accurately as the sensor means and the control electronics would allow.

Addition of the sensor means would allow certain amount of fault-correction. For example, if susceptor rotation is slower than expected as measured by the sensing means, then control electronics can alert a human operator or temporarily increase the drive currents m of the solenoids to try to compensate for any additional loads or changes in the device. Operation of the device according to the invention can be made even more reliable with the sensors in place. 5. Planetary Device

FIGS. 10a, 10/3, and 10c show a planetary form of susceptor rotation according to the invention. A main susceptor 14 with three ferromagnetic elements llα, lib, and lie rotates parallel to a reference surface 30 about a shaft 20 perpendicular to this surface. Main susceptor 14 is held in sustentation by the action of gas flow through gas inlets 40α, 40/3, 40c into sector-shaped depressions or grooves 60α, 60/3, and 60c. Main susceptor 14 is caused to rotate in the indicated directions by interactions between ferromagnetic elements llα, 11/3, and lie and a magnetic actuating assembly 70 containing 48 magnetic solenoids 74 and optionally 48 sensor means 82. A cyhndπcal-reactor-chamber wall 90 is also shown

Ma susceptor 14 also includes a number of secondary susceptors 114, 214, and 314 on a reference surface 130 facing away from reference surface 30 Secondary susceptor 114 is used as an example applicable to secondary susceptors 214 and 314 Secondary susceptor 114 contains six ferromagnetic elements lllα, lllfo, 111c, llld, llle, and 111/ Secondary susceptor 114 is held in sustentation by the action of gas flow through an additional gas inlet 42 in reference surface 30, a system of gas conduits 43, and a gas inlet 140 into an annular groove 162 connected to several sector-shaped depressions or grooves 160α, 160fc, and 160c on reference surface 130 Similarly, secondary susceptors 114, 214, and 314 are caused to rotate in the indicated directions by interactions between their respective ferromagnetic elements and magnetic actuating assembly 70

Understanding and utilization of this planetary device is aided by the letters P, X, Y, and Z as indicated on the individual magnetic solenoids within magnetic actuating assembly 70 Individual magnetic solenoids 74 are controlled by control electronics 80 Optional sensors 82 are also connected to control electronics 80

At the moment in time as illustrated, solenoids 74 marked with P are responsible for the rotation of main susceptor 14, those marked with X for secondary susceptor 114, those marked with Y for secondary susceptor 214, and those marked with Z for secondary susceptor 314 Current waveforms illustrated in FIGS 8α or Sb can be applied to these solenoids with minor modifications easily done by those well-versed in the art The positions of these letters P, X, Y, and Z shift in time to correspond to the angular orientation of mam susceptor 14 The rotation rates of individual susceptors 14, 114, 214, and 314 can be independently controlled in this fashion, thereby allowing for extremely complicated rotation patterns With the addition of optional sensors 82, rotation rates and angular orientation of individual susceptors can be confirmed and measured, allowing for possible immediate fault-correction by control electronics 80 or a human operator

Summary, Ramification, and Scope of The Invention

Accordingly, versatile devices comprising one or more rotatable susceptors or platforms and means for obtaining individual rotation of the susceptors or platforms have been described Many constraints of prior-art devices have been eliminated

(a) No constraint on minimum rotation rate

(b) Escaping gases that disturb the gas flow over the susceptor are minimized

(c) Rotation rates of platforms can be determined essentially exactly

(d) Perfect matching of rotation rates between individual platforms in the planetary device is an inherent characteristic

(e) The process of starting and stopping rotation generates no contaminating particles

(f) Synchronization of rotation with external events is simple

While my above descπpϋon contains many specificities, these should not be construed as limitations on the scope of the invention, but rather as exemplifications of preferred embodiments thereof Many other variations are possible. For example, the ferromagnetic elements may be square or oval The susceptor or platform may be a square, a polygon, or shaped to fit a particular workpiece Instead of being shaped like sectors, the grooves on the reference surface can be stripes, patches, concentric annular πngs, or many other combinations to achieve similar effects in susceptor sustentation The design of the magnetic actuating assembly allows an enormous amount of freedom to adapt to specific applications, such as water-cooling for magnetic solenoids, integration of solenoids into the reactor-chamber wall, or improved magnetic circuits to allow for better efficiency.

Accordingly, the scope of the invention should be determined not by the embodiments illustrated, but by the appended claims and their legal equivalents.

Claims

Claims: I claim

1 A device comprising a reference surface, a rotatable platform, said platform being supported from said reference surface by bearing means, at least one ferromagnetic element integrated into said platform, a magnetic actuating means, wherein rotary movement of said platform is obtained by forces of magnetic interaction between said element an said actuating means

2 A device according to claim 1, wherein said platform includes a number of secondary platforms, said secondary platforms include at least one ferromagnetic element integrated into each said secondary platform wherein rotary movements of said secondary platforms are obtained by forces of magnetic interaction between said elements of said secondary platforms and said actuating means

3 A device comprising a reference surface, a rotatable susceptor, said susceptor being supported from said reference surface by bearing means, at least one ferromagnetic element integrated into said susceptor, a magnetic actuating means, wherein rotary movement of said susceptor is obtained by forces of magnetic interaction between said element and said actuating means

4 A device according to claim 3, wherein said susceptor includes a number of secondary susceptors, said secondary susceptors include at least one ferromagnetic element integrated into each said secondary susceptor, wherein rotary movements of said secondary susceptors are obtained by forces of magnetic interaction between said elements of said secondary susceptors and said actuating means

5 A device comprising a reference surface, a rotatable platform, said platform being supported from said reference surface by rotary bearing means, at least one ferromagnetic element integrated into said platform, said element being of a volume smaller than sai platform, a magnetic actuating means, wherein rotary movement of said platform is obtained by forces of magnetic interaction between said element an said actuating means

6. A device according to claim 5, wherein said platform includes a number of secondary platforms, said secondary platforms include at least one ferromagnetic element integrated into each said secondary platform wherein rotary movements of said secondary platforms are obtained by forces of magnetic interaction between said elements of said secondary platforms and said actuating means

7. A device comprising a reference surface, a rotatable susceptor, said susceptor being supported from said reference surface by rotary bearing means, at least one ferromagnetic element integrated into said susceptor, said element being of a volume smaller than said susceptor, a magnetic actuating means, wherein rotary movement of said susceptor is obtained by forces of magnetic interaction between said element and said actuating means.

8. A device according to claim 7, wherein said susceptor includes a number of secondary susceptors, said secondary susceptors include at least one ferromagnetic element integrated into each said secondary susceptor, wherein rotary movements of said secondary susceptors are obtained by forces of magnetic interaction between said elements of said secondary susceptors and said actuating means.

9. A device comprising a reference surface having at least one fluid inlet, a rotatable platform, said platform being supported from said reference surface by flow of fluid from said inlet, a rotary shaft means to prevent lateral movement of said platform, at least one ferromagnetic element integrated into said platform, a magnetic actuating means, wherein rotary movement of said platform is obtained by forces of magnetic interaction between said element and said actuating means, whereby said platform can be maintained at a separation distance h greater than zero from said reference surface by said flow of fluid.

10. A device according to claim 9, wherein said platform includes a number of secondary platforms, said secondary platforms include at least one ferromagnetic element integrated into each said secondary platform, wherein rotary movements of said secondary platforms are obtained by forces of magnetic interaction between said elements of said secondary platforms and said actuating means.

11. A device comprising a reference surface having at least one fluid inlet, a rotatable susceptor, said susceptor being supported from said reference surface by flow of fluid from said inlet, a rotary shaft means to prevent lateral movement of said susceptor, at least one ferromagnetic element integrated into said susceptor, a magnetic actuating means, wherein rotary movement of said susceptor is obtained by forces of magnetic interaction between said element and said actuating means, whereby said susceptor can be maintained at a separation distance h greater than zero from said reference surface by said flow of fluid. 12 A device according to claim 1 1 wherein said susceptor includes a number of secondary susceptors. said secondary susceptors include at least one ferromagnetic element integrated into each said secondary susceptor, wherein rotary movements of said secondary susceptors are obtained by forces of magnetic interaction between said elements of said secondary susceptors and said actuating means

13 A device comprising a reference surface having at least one fluid inlet, a rotatable platform, said platform being supported from said reference surface by flow of fluid from said inlet, said platform being supported laterally also by flow of fluid from said inlet to prevent lateral movement of said platform, at least one ferromagnetic element integrated into said platform, a magnetic actuating means, wherein rotary movement of said platform is obtained by forces of magnetic interaction between said element an said actuating means, whereby said platform can be maintained at a separation distance h greater than zero from said reference surface by said flow of fluid

14 A device according to claim 13, wherein said platform includes a number ot secondary platforms, said secondary platforms include at least one ferromagnetic element integrated into each said secondary platform wherein rotary movements of said secondary platforms are obtained by forces of magnetic interaction between said elements of said secondary platforms and said actuating means

15 A device comprising a reference surface having at least one fluid inlet, a rotatable susceptor, said susceptor being supported from said reference surface by flow of fluid from said inlet said susceptor being supported laterally also by flow of fluid from said inlet to prevent lateral movement of said susceptor, at least one ferromagnetic element integrated into said susceptor, a magnetic actuating means, wherein rotary movement of said susceptor is obtained by forces of magnetic interaction between said element and said actuating means, whereby said susceptor can be maintained at a separation distance h greater than zero from said reference surface by said flow of fluid

16 A device according to claim 15, wherein said susceptor includes a number of secondary susceptors, said secondary susceptors include at least one ferromagnetic element integrated into each said secondary susceptor, wherein rotary movements of said secondary susceptors are obtained by forces of magnetic interaction between said elements of said secondary susceptors and said actuating means